How did they get there? The colonization of a hydrothermal vent after volcanic eruption

To some people, a volcanic eruption means “Ahh! Run! Hot Lava!” But to others, it means “SCIENCE!” To those studying hydrothermal vent communities, that is (and a wide berth of geologists).

Hydrothermal vents are cracks in the seafloor formed when tectonic plates spread apart, which spew out hot, mineral-rich water from the interior of the earth. Thus they are most commonly found on sea ridges, such as the Mid-Atlantic Ridge and the East Pacific Rise, where two or more tectonic plates meet and clash underwater. These vents host exotic communities of organisms. All the hot, mineral-rich water attacts chemosynthetic bacteria, or bacteria that get their energy from chemicals instead of the sun (as there is no sunlight on the seafloor), which in turn attract organisms that graze on the bacteria.

An important organism in many hydrothermal vent communities is the tube worm. It can grow a couple meters long and lives inside of a tube that it builds for itself out of chitin, into which they can retreat in case predators are around. The weirdest thing about tube worms: they have no mouth or digestive tract! For their food, they require a symbiotic relationship with chemosymbiotic bacteria, drawing their nutrients from the bacteria, presumably in return for the nice home.

A hydrothermal vent in the Pacific Ocean. You can see tube worms (red worms in white tube) in the foreground, and the chemical-rich water (hydrogen sulfide, in this case) shooting off like a plume of smoke in the background.

This is just one of the many strange and diverse organisms found in hydrothermal vent communities. There have been over 300 new species identified since the first vent was discovered in 1977. However, due to their nature, these vents and their communities are ephemeral: just as easily as they are created by the spreading of the earth’s plates, they are just as easily closed off. Once the mineral-rich water is gone, so are the bacteria, causing many of the species inhabiting the vent (including tube worms) to go locally extinct.

These communities present an interesting question to biologists: from where do these communities come? The two dominant hypotheses are: (1) there is a well-mixed pool of larvae that colonize new vents (similar to the “everything is everywhere” hypothesis about microbial distribution I pondered here), and (2) vent communities are created from larvae supplied by local populations through migration. (Side note: Marine dispersal has been a hot topic on Research Blogging this week! I recommend this post by the recently-graduated LabRat (Congratulations!) on vertical distribution of microbes by hitchhiking on plankton, as well as this post on Southern Fried Scientist about how “ghost populations” affect marine migration.)

A group of scientists from Woods Hole had been studying a number of vents along the East Pacific Rise, a huge ridge cutting across the center of the Pacific, when they noticed that one of their communities had disappeared! Lava from an underwater volcanic eruption had paved over the vents and their communities, killing off species in almost the entire area (RIP). But this lava did not clog all the vents: some of the vents (as well as freshly created ones) continued to spew the hot, mineral-rich water. As the scientists had collected data on the local pool of larvae and the pre-eruption community, it presented a perfect opportunity to study marine dispersal by comparing the vent communities in this area before and after the eruption. If there is a general pool of larvae, they expected a similar community before and after the event. A distinct community post-eruption would signal local migration.

What do these species look like? Mostly gastropods. I also included the two different tubeworm species the scientists found at the vent communities. Most images from http://www.whoi.edu/vent-larval-id/GastSize.htm. Click image for larger view!

In their 2010 PNAS paper, the scientists found that both the larvae found in the vicinity and the species that settled to colonize the vent area differed drastically from those found before (see figure below). The dominant tubeworm species was Tevnia jeichonana, replacing Riftia pachyptia (see figure above for images). Most interesting was the arrival of Ctenopelta porifera, which had never been found at the site before – the nearest known population is 300 km away! These data suggest that, at the least initial community, arrives through the second hypothesis: supply through local populations and not a “well-mixed, time-invariant larval pool.”

The colonizers (warm and hot vents) and larvae differed significantly before and after the eruption. Click image for larger view.

There are some possible reasons for this. The new vents could be spewing water that has a different biochemical makeup, supporting a distinct species of bacteria and thus a distinct community of colonizers. The authors hadn’t done chemical analysis yet (which would have made it a stronger paper, in my opinion), but offered this as a possibility. Additionally, as I mentioned above, this is just the initial community. The authors found T. jerichonana as the dominant tubeworm species, which they have seen replaced by R. pachyptila (the previous dominant species) and later by the mussel Bathymodiolus thermophilus at other vents over time. This creates the possibility that these vents are colonized initially through local populations and after they are “broken in” and deemed habitable, other more robust species move in, ousting the previous colonizers. Where these species come from, either from a well-mixed pool or local populations, only time will tell.

And that’s what’s so great about ecology: this experiment isn’t over! The scientists are surely continuing to collect data on these vent communities, and over the decades we will be able to follow them through their succession. So keep your eyes open for the next paper from these scientists to hear the rest of the story…

A hydrothermal vent in the Pacific Ocean. You can see tube worms (red worms in white tube) in the foreground, and the chemical-rich water (hydrogen sulfide, in this case) shooting off like a plume of smoke in the background.